The present invention relates to an interface circuit for protecting a telephone from overcurrents flowing in continuously due to transient overvoltages such as surge voltages or mixing etc. of mains power lines and subscriber lines.
A protection circuit is provided at an interface of a telephone and subscriber lines because of the possibility of a transient indirect lightening stroke accompanying a lightening strike being propagated to subscriber lines hanging in space or the possibility of overcurrents continuously flowing over a long period of time to a certain extent due to mixing with a mains power line. Configurations such as, for example, connecting a varistor element between two subscriber lines or connecting a varistor element between a subscriber line and earth are well-known as lightening surge countermeasures. When a transient surge voltage exceeding the varistor voltage is applied to a subscriber line, the surge voltage is absorbed as a result of the varistor element making a transition to conducting mode, and a speech circuit within the telephone is protected.
Further, configurations where, for example, a PTC thermistor (Positive Temperature Coefficient Thermistor) is interposed at an interface between a subscriber line and a telephone are also well known as a countermeasure for heating and combustion of a telephone due to mixing of subscriber lines and mains power lines. When an overcurrent flows into a PCT thermistor continuously over a certain period of time, input impedance of the interface increases in accompaniment with rise in element temperature and flowing in of overcurrents to within the telephone can be suppressed.